Less than 1% of the total body magnesium is present in the peripheral blood. We developed an assay to determine the magnesium content and concentration of blood mononuclear cells as an index for intracellular magnesium and/or total body magnesium status. We determined plasma, red blood cell and mononuclear blood cell magnesium concentrations in normal individuals between the ages of twenty and seventy. We determined these same magnesium parameters in women with the premenstrual syndrome and the control group at four points during the menstrual cycle. The results show magnesium parameters do not differ with age. The magnesium concentration of red blood cells decreases significantly during the menstrual cycle in women with the premenstrual syndrome.